This invention was made during the course of, or under, a contract with the U.S. Atomic Energy Commission.
This invention relates generally to numerical controllers for automatic machining and more specifically to improvements in interpolator circuits of the digital differential analyzer type for use in numerical controllers to generate drive motor pulses to produce precisely defined, non-linear machine tool motion from a single data block of part description data.
In the art of numerical controlled automatic machining digital differential analyzers (DDA) are used to perform mathematic functions from part description data blocks to generate motor drive pulses which position the machine slides. One block of data, which may be read from punched tape or other conventional storage means, essentially specifies the end points for a given tool motion. The data block is read into a buffer storage register in the conventional numerical controller and the controller generates command pulses which are fed to the machine slide drive motors. The output circuits of a numerical controller (NC) are generally special DDA circuits (one for each axis of movement) which accept data in the form of a binary number which specifies the tool motion end points. The DDA is said to interpolate between the end points for a data block. Most NC machines are capable of only straight-line tool motion or linear interpolation; some provide coarse non-linear interpolation capability.
If only linear interpolation is available, all contours to be cut must be approximated by straight-line segments. The segments may be made short enough so that any reasonable approximation may be obtained. The typical allowed deviation from the true contour is one-half the machine pulse resolution. Each straight-line segment is represented by one block of data on the input tape. A difficulty arises from the latter fact. Very close tolerances can result in very long data tapes--so long that tape reader errors become highly probable. Error rates of one error per million bit transferred are typical for tape readers. Since the standard BCD tape format has 80 bits per inch, the error rate works out to about one error per 1000 feet of tape. The result is that, if a part requires a tape of about 1000 feet or more in length, the risk of machining failure is high.
Tape length for machining large or complicated parts to close tolerance or machining parts which require many specialized operations have steadily increased over the years until lengths of 1000 feet of more are not unusual. Machining failures have occurred because of tape reader errors, and ways have been sought to eliminate the problem. One technique which, aside from its other advantages, will solve the problem is direct digital numerical control (DDNC). Local core memory at the machine tool is loaded with the entire file of part description data from a remote computer via data transfer lines. The stored information may be checked for errors before machining begins. The data are then read from the core as machining proceeds. There is still a problem in the core memory approach, however. The increasingly large volumes of data require large blocks of core memory, which are expensive. The core presently costs about $1,800 per 8K module, and up to 8 modules may be required in extreme cases for a total core memory cost of about $14,400. Thus, there is a need for some way to reduce the volume of data required by the controller.